The prior art teaches a joint structure of an industrial robot in which an arm is interconnected through a parallel linkage having four or more links (see for example Japanese Laid-Open Patent Application No. Hei 10-296680 (paragraphs 0002, 0008 and 0012, and FIGS. 2, 3, 5 and 6).
A robot joint structure that interconnects links by a single axis is also known. From the viewpoint of safety and protection against dust, it is ordinarily preferable for a robot to have the outside of the links covered by covers so as not to expose the internal structure. A joint structure has therefore been proposed for a legged mobile robot, for example, in which links (e.g., a thigh link and a shank link) are interconnected by a single axis (i.e., the thigh link and shank link are directly connected without use of an intermediate linkage), the edge of the cover covering one link is formed to have a spherical surface centered on the single axis and the edge of the cover covering the other link is given a concave shape corresponding to the spherical surface, whereby no gap arises between the covers when the joint is moved (see Japanese Laid-Open Patent Application No. 2002-210682 (FIG. 4)).
In stationary industrial robots used for various tasks in plants and the like, the need to expand the range of motion of a task-performing hand, so as to increase the size of the reachable space, and the need to increase the critical or limit value of the driven speed can be met by appropriately defining the number of joints between the main unit and the task-performing hand, the arm (linkage) length and the driving power of the actuators. In contrast, in a legged mobile robot, particularly a humanoid robot or the like modeled after the form of the human body, design factors such as the number of joints and the link lengths are subject to greater restriction than in the case of an industrial robot owing to appearance and functional considerations. Moreover, autonomous robots are also limited with regard to usable actuators owing to power consumption, available mounting space and other considerations. In an autonomous legged mobile robot or the like therefore, expansion of the range of motion of the arms and legs, for example, and increase of the critical value of the driven speed have to be achieved by increasing the range of motion (angle of rotation) and increasing the critical value of the driven speed of the individual joints of the arms and legs.
As shown in FIG. 23, in a legged mobile robot equipped with ordinary single-axis joints, in order to avoid physical interference in the links 102 and 104 or covers covering them, their rotation axis 100 is sometimes offset outward from the center of links. This outward offsetting of the rotation axis lowers the likelihood of interference in the links and covers on the offset side and increases the range of movement.
In an articulated robot, however, as shown in FIG. 24, a posture in which their rotation axes (joints) 110, 112 and 114 are positioned on the same line constitutes a singularity posture. Since control diverges or oscillates when a robot assumes a singularity posture, the angle of rotation of the joints has to be constricted so that the singularity does not occur. In the elbow joint of the arm of a humanoid robot, for example, the elbow joint (corresponding to the rotation axis 112) has a range of motion between a slightly bending angle and the maximum bending angle.
Here, if, as shown in FIG. 23, the rotation axis 100 is offset outward for avoiding physical interference in the links and covers, it is necessary, as shown in FIG. 25, to establish an angle of rotation θos between the state in which the links 102 and 104 are fully extended (state of the joint being driven as far in the extending direction as the mechanism permits) and the state in which the rotation axes 100, 106 and 108 are located on the same line, i.e., the singularity posture. The range of motion (angle of rotation) of the rotation axis 100 that can be utilized in control is therefore the range of motion determined by the mechanism, minus the angle of rotation θos. The angle of rotation θos increases with increasing amount of offset of the rotation axis 100. The prior art therefore involves the inconvenience that when the rotation axis is offset in order to expand the range of motion in the bending direction, the range of motion in the direction of extension is markedly constricted and reduced.